In recent years, dry film resists have been widely used in the field of print board making. There are two background art processes for the preparation of dry film resists; that is, tenting and plating processes. In both of these processes, techniques for making fine patterns have been improved rapidly and, as a result, great concern has been directed toward the development of a dry film resist having a higher level of resolution. With the aim of satisfying such a demand, dry film resists capable of yielding high resolution levels have been put on the market.
For example, attempts have been made to obtain high resolution levels by thinning down the thickness of support films and photopolymerizable resin layers. Though thinning of a support film can improve the resolution to a certain degree, such a thinning process has a workability problem in that the thin support film is apt to be torn-off at the time of support peeling. Thinning of a photopolymerizable resin layer is also effective for improving the resolution, but has disadvantages in that a tented membrane becomes weak and a thick plating cannot be effected.
It is also known that high resolution levels can be obtained by improving the photopolymerizable resin layers. For example, a high level of resolution can be effected by making use of a photopolymerizable resin layer. Such has been disclosed by the present inventors in U.S. Pat. No. 5,030,548. Certainly, a higher level of resolution than that of the background art dry film resist can be stably obtained when such a photopolymerizable resin layer type is used (at least at a small prototype production scale). In a large scale production line, however, exposure, development and like conditions are not always constant because of the production of a considerably large number of boards. Illustrative examples of such irregular conditions include a decrease in the quantity of light due to the deterioration of the light source during an exposure step and changes in the development load due to the changed exhaustion degree of a developing solution during a development step. In such steps (in which momentary changes in these conditions are unavoidable), it is difficult to maintain a maximum resolution level by constantly controlling optimum conditions.
Consequently, it is important to provide a stable high resolution level for the purpose of satisfying the recently increasing demand for high resolution levels. That is, it is especially important for a dry film resist to be able to provide a high level of resolution over a broad exposure and development controlling range.
On the other hand, in the general case of the background art dry film resist, a resist pattern is formed on a board by laminating the resist on the board, effecting exposure through a support, peeling the support from the board and then developing the pattern. In such a process, the support peeling step is carried out by hand, thus entailing an increase in the personnel expenditure.
With the aim of overcoming such a problem by providing a support which can be peeled automatically by a machine and, at the same time, improving the resolution to a relatively higher level, JP-A-2-213849 (the term "JP-A", as used herein, means an "unexamined published Japanese patent application") proposes a process in which a specific polyvinyl alcohol intermediate layer made of a saponification product of a polyethylene glycol/vinyl acetate graft polymer is interposed between a support and a photopolymerizable resin layer, and the support is peeled off prior to the exposure step.
In addition, with the aim of achieving the same purpose, JP-A-63-197942 discloses the use of a carboxylated polyvinyl alcohol as an intermediate layer. It is considered in general that, for the purpose of shuttingoff oxygen which inhibits photopolymerization, polyvinyl alcohols are desirable as water soluble polymers for use in such an intermediate layer (which is used in combination with a photopolymerizable resin layer).
These polymers, however, are not realistic because they are particular modified polymers, they cannot be obtained easily in general, and they have a high production cost. Also, being high in hygroscopicity and low in Tg (glass transition point), these polymers have a strong tendency to increase the peeling strength of the support during a period between the lamination and support-peeling steps. An increase in the peeling strength makes it difficult to effect automatic peeling and, in some cases, finally results in such a condition that the support is split into fragments which remain on the board side.
When the board side portion remains as it is after peeling of the support, so-called "reticulation" is apt to occur in which the intermediate layer swells by absorbing moisture and presses its lower photopolymerizable resin layer subsequently causing formation of a network distortion which spoils transparency of the resin layer. Generation of such a reticulation phenomenon deteriorates the resulting resolution considerably.
In addition, U.S. Pat. No. 3,884,693 discloses gelatin, polyvinyl alcohol, polyvinyl pyrrolidone and like polymers to be used as the intermediate layer. Each of these polymers, however, has the aforementioned disadvantages and, therefore, is not satisfactory in obtaining a practically useful intermediate layer.
Also, JP-B-46 2121 (the term "JP-B", as used herein, means an "examined Japanese patent publication") discloses celluloses, carboxylalkyl starch, ethylene oxide polymers and the like as the source of the intermediate layer, in addition to the aforementioned polymers. However, the specification and examples of this patent publication do not describe the effects of these materials as a solution to various problems involved in the aforementioned applications.